pmb.aportgen.gcc: remove isl from depends and set _libgcc=false

isl was removed in Alpine, remove it to fix #2167 (closed).

Alice wrote in #2167 (closed):

to recap a little- isl was the wrong depends= there because it was isl0.18, and not what isl-dev actually pulled and linked to (even ~2 years ago it was 0.22), so if you were relying on the .so linking detection, you don't need the depends= at all. otherwise, have to bump the number

Set _libgcc=false to fix #2168 (closed).

Upgrade the testdata gcc aport to the version currently in alpine edge, as the previous one didn't have the _libgcc variable.

See commit messages for details.

Reading individual commits is recommended since the one that updates the testdata completely changes the APKBUILD.